This study aimed to characterise preservice teachers' (PSTs') noticing in a mathematics classroom and its influence on their lesson modification. Written narratives on video-taped lesson observation were analysed qualitatively in two components, attending and reasoning. The findings indicate that PSTs' initial noticing focused on general instruction-learning strategies and considered students as a main agency of the classroom activities. PSTs' reasoning of their initial noticing and responsiveness was grouped into four categories: interpretive, descriptive, evaluative and noveltive reasoning. These different types of reasoning explained how they used their analysis of noticing to make a decision on lesson modification. Findings suggest that PSTs need opportunities to analyse practices with articulated vocabulary and framework in order to develop meaningful noticing involving attentive, analytic, and responsive abilities.